The present invention relates to improved transparent conjugated diene/monovinyl-substituted aromatic block copolymers and the process of their production. In one aspect, the present invention relates to improved compositions composed of a mixture of transparent conjugated diene/monovinyl-substituted aromatic block copolymers and thermoplastic polymers such as polystyrene. In a particular aspect, the present invention relates to a process for the preparation of linear transparent conjugated diene/monovinyl-substituted aromatic block copolymers which do not contain coupling agents.
Various transparent conjugated diene/monovinyl-substituted aromatic block copolymers are known. However, these copolymers have deficiencies in certain mechanical properties and, in particular, do not have a good balance of mechanical properties when they are mixed with known thermoplastic polymers.
Resinous block copolymers having at least three branches and having a multimodal structure are described in Kitchen et al, U.S. Pat. No. 3,639,517. The copolymers prepared according to Kitchen et al, have a relatively low impact strength and a relatively low elongation, alone or when admixed with polystyrene. In addition, the optical properties and flexibility of these copolymers are not exceptionally good.
An improvement in the flexibility properties has been described in Kitchen, U.S. Pat. No. 4,091,053 but other defects have not yet been improved. The flexibility properties of the copolymers, according to Kitchen, are improved by the accurate adjustment of the different styrene additions during preparation.
Branched block copolymers having at least two branches which contain elastomeric end blocks of the A.sub.1 -B.fwdarw.A.sub.2 (x)A.sub.2 .rarw.B and (A.sub.1 -A.sub.2 -B.fwdarw.A.sub.3)(A.sub.3 .rarw.B-A.sub.2) type, wherein the transition in the elastomeric block occurs gradually are described in Fahrbach et al, U.S. Pat. No. 4,086,298 and Fahrbach et al, U.S. Pat. No. 4,167,545 respectively. However, block copolymers produced according to these two patents do not have very good mechanical properties.
Transparent block copolymers of bimodal structure that have at least two elastomeric branched end groups and butadiene/styrene random copolymers are described in Bi et al, U.S. Pat. No. 4,221,884. However, the block copolymers produced according to this patent do not provide satisfactory mechanical properties either.
Linear block copolymers containing an elastomeric block of butadiene/styrene random copolymer are described in Miki et al, U.S. Pat. No. 4,089,913. However, as above, block copolymers produced according to this patent do not give good results, particularly when mixed with polystyrene.